Use at for the time:
"I always wake up at 7am."
"Let's meet at lunchtime for a coffee."
Use on for days and dates.
"It's his birthday on Sunday."
"The conference is on 10 March."
Use in with weeks, months, years and seasons.
"She retired in May."
"I'll be back in 3 weeks."
"Paul was born in 1990."
"The weather in spring is beautiful."
Use for with a period of time / length of time.
"I've been living here for 6 months."
Use since with the start of a period of time.
"I've been living in Brighton since January."
Use by when we want to say "not later than" or any time until this point (a deadline).
"My mother said I have to be home by 7pm."